How to take pictures from the GE?
Pose your weapon according to the view , than run the script to take a picture using openGl render , got o the UV editor and the image will have the name of the active object :

import bpy

# path to the folder
file_path = bpy.data.filepath
file_name = bpy.path.display_name_from_filepath(file_path)
file_ext = '.blend'
file_dir = file_path.replace(file_name+file_ext, '')

mainScreen = bpy.context.screen

#current scene
scene = mainScreen.scene 
#set render settings
scene.render.resolution_x = 128
scene.render.resolution_y = 128
scene.render.resolution_percentage = 100

#render from view (set view_context = False for the camera render)
bpy.ops.render.opengl(view_context = True)

#save and load the render (you can't keep the render result)
img_name = bpy.context.object.name+".png"
bpy.data.images['Render Result'].save_render(file_dir+img_name)
bpy.ops.image.open(filepath = file_dir+img_name)
bpy.data.images[img_name].pack()



you can change the render size in the script




For screenshots while playing the BGE has a function for that :
bge.render.makeScreenshot(filename)    see the DOC